Objective: Studies include the evaluation of platelet storage, the effect of platelet transfusions on infection, the effect of incompatible platelet transfusions on the risk of infection, the isolation of leukopoietic factors and regulatory substances elaborated during leukopheresis and the investigation of febrile transfusion reactions in donors undergoing leukopak leukopheresis. Animal studies include the effect of prophylactic granulocytes on clearance of experimentally induced infection. Approach: Plasma obtained following pheresis in tested in vitro for its effect on colony forming stem cells and in vivo for its ability to cause release of granulocytes and stimulate proliferation of precursor cells. An animal model simulating human granulocyte procurement by filtration leukopheresis is used to study experimentally induced sepsis and to determine the role of granulocyte transfusions in this setting.